In a powered switch circuit, a load (e.g., lamp, motor, circuit component, computer, or the like) is connected to a power source through a switch. When a short circuit or current surge occurs on the load, or the switch is powered into a short circuit or heavy load, an excessive current may flow through the switch. This is referred to as an overcurrent condition. These overcurrent conditions can present as voltage spikes with durations ranging from a few microseconds to hundreds of milliseconds. Overcurrent conditions can damage the switch, damage the power source, cause a voltage transient, or result in damage or malfunction to other connected circuitry. Overcurrent conditions can be caused by a number of factors, such as, for example, inserting or removing loads under operating conditions (sometimes referred to as “hot swapping”). Overcurrent protection circuits may be used to shut off a switch and protect against overcurrent conditions.
Various kinds of overcurrent protection methods have been proposed. For example, FIG. 1 illustrates an overcurrent protection apparatus 100. The overcurrent protection apparatus 100 includes a switch 110 having a MOSFET transistor 112 and a Diode 114. Apparatus 100 also includes terminals 122 and 124 for connecting a source (not shown) to a load (not shown). Furthermore, a control circuit 130 comprised of an operational amplifier is configured to open and close the switch 110. As depicted, the inputs to the control circuit 130 are connected to the terminals 122 and 124 (or the source and the drain of the MOSFET transistor 112.) Therefore, the control circuit 130 is configured to measure the voltage drop between the drain and the source of the MOSFET transistor 112 and compare this voltage drop to a built in voltage differential. The output of the control circuit 130 is connected to the gate of the MOSFET transistor 112 and shuts off the MOSFET transistor 112 (thereby closing the switch 110) if an overcurrent is detected as measured by the voltage drop.
FIG. 2 illustrates another overcurrent protection apparatus 200. Apparatus 200 includes a switch 210 having a MOSFET transistor 212 and a Diode 214. Apparatus 200 also includes terminals 222 and 224 for connecting a source (not shown) to a load (not shown). Furthermore, a control circuit 230 comprised of an operational amplifier is configured to open and close the switch 210. Apparatus 200 also includes a first resistor 242 (R1) connected in series between terminal 224 and the switch 210. A second resistor 244 (R2) connected in series between the terminal 224 and a constant current source 252. The inputs to the operational amplifier are connected across the first and second resistors 242, 244. As such, the control circuit 230 is configured to compare the voltage drop across the first resistor 242 to the voltage drop across the second resistor 244 and turn the switch 210 on or off accordingly.
The current limiters apparatuses described in FIGS. 1-2 have various disadvantages. For example, apparatus 100 and apparatus 200 merely check for an overload threshold condition and shuts the switch off once such a condition occurs. Furthermore, FIG. 2 adds a resistor in series between the source and the load, thereby adding an extra voltage drop into the current path.
FIG. 3 illustrates another overcurrent protection apparatus 300. Apparatus 300 is described in greater detail in U.S. Pat. No. 6,917,503 entitled “Programmable current limiting using a shunt resistor,” which reference is incorporated entirely herein by reference. Apparatus 300 includes a switch 310 having a MOSFET transistor 312 and a Diode 314. Apparatus 300 also includes terminals 322 and 324 for connecting a source (not shown) to a load (not shown). Furthermore, a control circuit 330 comprised of an operational amplifier is configured to open and close the switch 310. Apparatus 300 also includes a first resistor 342 (R1) connected in series between the terminal 324 and a constant current source 350. The inputs to the operational amplifier are connected between the first resistor 342 and the current source 350 and at the terminal 322. Accordingly, the control circuit 330 is configured to compare the voltage drop across the first resistor 342 to the voltage drop across the MOSFET transistor 312 and turn the switch 310 on and off accordingly.
Apparatus 300 seeks to compensate for temperature fluctuations. More specifically, the on-resistance of the MOSFET transistor 312 is proportional to the absolute temperature. The current source 350 can be made to behave similar to the on-resistance of the MOSFET transistor 312, that is, proportionally to the absolute temperature. These two temperature coefficients compensate each other. However, as will be appreciated, the first resistor 342 is also temperature dependent, which affects the temperature dependent balance between the MOSFET transistor 312 and the current source 350. As such, the voltage drop of the first resistor 342 is not proportional to the absolute temperature.
Another disadvantage to apparatus 200 and apparatus 300 shown above is the current source (e.g., 252 or 350) is dependent on the power supply. As will be appreciated, most current sources are constant when the connected power supply is constant. However, when the voltage level of the power supply increases, the current level of the current source also increased. This can affect the ability of the control circuits to measure the voltage drops. Especially when the power supply has a large swing or when the on-resistance of the MOSFET is very small.
Thus, there is a need for a current limiter that provides temperature compensation. Also there is a need for a current limiter that is not affected by voltage swings of the power supply.